Various methods are known for the manufacture of hollow plastic articles (e.g. bottles, cans and other containers). In a first group of such methods, known as extrusion blow-molding, the individual hollow articles are manufactured from a thermoplastic tube, which is drawn to length and blown in a mold. In another group of such methods, a preform is blown, but in this case blowing takes place in a cavity that defines the final shape of the hollow article in question. This group of methods, to which the present invention relates, includes injection blow-molding as well as stretch blow-molding, the preform being mechanically stretched before it is blown in the case of stretch blow-molding.
Multilayer preforms are used to manufacture high-grade hollow plastic articles having a multilayer structure. The hollow multilayer plastic articles manufactured from multilayer preforms are characterized by increased functionality, for example by the fact that a first layer of the will of the hollow article is optimized with respect to mechanical strength, while a second layer is optimized with respect to chemical resistance to the intended contents of the hollow article and/or with respect to the surface quality, i.e. low porosity.
Extensive proposals for the manufacture of multilayer preforms have indeed been published (for example, see DE 3049725 C2, WO 8903756 A1, JP 4104022 B2, WO 2011006999 A1, WO 2013000044 A2, U.S. Pat. No. 4,904,509 A, U.S. Pat. No. 8,518,504 B2, U.S. Pat. No. 5,972,258 A, U.S. Pat. No. 6,276,914 B1, U.S. Pat. No. 6,344,249 B1). Nevertheless, the manufacture of particularly high-grade hollow multilayer plastic articles from multilayer preforms continues to be a technical challenge, which has not yet been solved satisfactorily. This is due among other aspects to the fact that the area of the material involved in the forming of the multilayer (preform into the hollow multilayer plastic article—by blow-molding or stretch blow-molding methods—is expanded by a factor of 12 to 15. The wall thickness of the multilayer preform is therefore reduced to as little as one fifteenth of the original thickness during blow-molding. Considering the small wall thickness of the hollow plastic article (which is desirable from the viewpoint of weight of the hollow plastic article and of the material costs incurred for the manufacture thereof) and the inhomogeneity—which cannot be ruled out in the use of conventional technology to manufacture the preforms—of material distribution within the individual layers of the multilayer preforms, this may have the consequence that the individual layers within the hollow multilayer plastic article manufactured from these preforms is functionally deficient, in the sense that they do not satisfy their intended function.